Tonoplast CBL–CIPK calcium signaling network regulates magnesium homeostasis in Arabidopsis

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-03, Vol.112 (10), p.3134-3139
• Main Authors: Tang, Ren-Jie, Zhao, Fu-Geng, Garcia, Veder J., Kleist, Thomas J., Yang, Lei, Zhang, Hong-Xia, Luan, Sheng
• Format: Article
• Language: English
• Subjects: Arabidopsis; Arabidopsis - genetics; Arabidopsis - metabolism; Arabidopsis Proteins - metabolism; Biological Sciences; Calcium Signaling; Calcium-Binding Proteins - metabolism; Cells; Homeostasis; Magnesium - metabolism; Mutation; Plant growth; Protein Binding; Protein-Serine-Threonine Kinases - metabolism; Proteins; Toxicity
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1420944112

Significance Plant growth requires a balanced supply of mineral nutrients. However, the availability of minerals varies constantly in the environment. How do plants adapt to low or high levels of minerals in the soil? The answer to this question holds the key to sustainable crop production. Mg is an essential macronutrient for plants, but high levels of Mg ²⁺ can become toxic. This study uncovered a regulatory mechanism, consisting of two calcineurin B-like (CBL) Ca sensors partnering with four CBL-interacting protein kinases (CIPKs) forming a CBL–CIPK network that allows plant cells to sequester the extra Mg ²⁺ into vacuoles, thereby protecting plant cells from high-Mg toxicity. To our knowledge, this report is the first that describes such a signaling mechanism for regulation of Mg homeostasis. Although Mg ²⁺ is essential for a myriad of cellular processes, high levels of Mg ²⁺ in the environment, such as those found in serpentine soils, become toxic to plants. In this study, we identified two calcineurin B-like (CBL) proteins, CBL2 and CBL3, as key regulators for plant growth under high-Mg conditions. The Arabidopsis mutant lacking both CBL2 and CBL3 displayed severe growth retardation in the presence of excess Mg ²⁺, implying elevated Mg ²⁺ toxicity in these plants. Unexpectedly, the cbl2 cbl3 mutant plants retained lower Mg content than wild-type plants under either normal or high-Mg conditions, suggesting that CBL2 and CBL3 may be required for vacuolar Mg ²⁺ sequestration. Indeed, patch-clamp analysis showed that the cbl2 cbl3 mutant exhibited reduced Mg ²⁺ influx into the vacuole. We further identified four CBL-interacting protein kinases (CIPKs), CIPK3, -9, -23, and -26, as functionally overlapping components downstream of CBL2/3 in the signaling pathway that facilitates Mg ²⁺ homeostasis. The cipk3 cipk9 cipk23 cipk26 quadruple mutant, like the cbl2 cbl3 double mutant, was hypersensitive to high-Mg conditions; furthermore, CIPK3/9/23/26 physically interacted with CBL2/3 at the vacuolar membrane. Our results thus provide evidence that CBL2/3 and CIPK3/9/23/26 constitute a multivalent interacting network that regulates the vacuolar sequestration of Mg ²⁺, thereby protecting plants from Mg ²⁺ toxicity.